Premalignant field often refers to histologically normal epithelial cells surrounding a tumor that carry some of the same genetic and/or epigenetic changes as in the tumor. Such cells and tumor cells surrounded by them may have a common clonal origin. A premalignant field can be formed upon its cell-of-origin acquiring a clonal growth advantage over its neighbor cells. Normally clonal competition between equipotent epithelial cells is neutral. However, some genetic mutations can tilt the neutral competition so that the mutated cells have increased ?fitness? and a higher chance to replace their neighbors, whereas other mutations can only do so upon interaction with environmental modifiers. A better understanding of how the interplay of genetic, epigenetic and environmental factors tilts the stochastic process of neutral clonal competition is the key for understanding how premalignant field is formed and how it can be targeted. p53 mutation is the most common mutation in human breast cancer and represents an early event in breast tumorigenesis. By inducing p53-loss in a small number of Keratin 8+ luminal mammary epithelial cells (MECs), we observed a premalignant field comprised of p53-deficient luminal MECs; mammary tumors later emerged from it with 100% penetrance. Since constitutive p53 knockout mice have a largely normal MEC phenotype, we hypothesize that this p53-deficient luminal premalignant field is formed upon interplay of p53-mutant luminal MECs and environmental factors (e.g., ovarian hormones, immune cells). As proliferation, differentiation and apoptosis of MECs are controlled by cyclic ovarian hormones, Aim 1 will determine if induced p53-deficiency in estrogen receptor (ER)+ or ER- luminal MECs triggers an imbalance of proliferation versus apoptosis between p53 mutant cells and their wild-type neighbors, resulting in a net accumulation of p53-deficent luminal cells over time, in an estrous cycle-dependent manner. Aim 2 will further determine the role of cyclic changes of ovarian hormones in establishing the p53-deficient luminal premalignant field, by ovariectomy, hormone (estrogen, progesterone) replacement, and tamoxifen treatment. Expression profiling of p53-deficient luminal MECs revealed a unique immune-related signature suggestive of immunosuppression; our preliminary study further demonstrated that M2-polarized macrophages could enhance growth of luminal MECs. Based on these data, Aim 3 will investigate potential roles of various immune cell types, in particular, macrophages (e.g., M2-polarized), in shaping the p53- deficient luminal premalignant field. Overall, a better understanding of how interaction of the hormone milieu and immune cells in the mammary gland with p53-mutant luminal MECs contributes to development of this p53-deficient premalignant field is expected to lead to novel strategies of breast cancer prevention, particular in high-risk populations (e.g., Li-Fraumeni syndrome patients). The idea and approach proposed here may also have broad-reaching implications in understanding premalignant fields in other cancer types.